This invention relates to the use of reaction products of hydrolyzed styrene-maleic anhydride copolymer and a dispersed hydroxy-alkyl modified starch as a constituent of a binding composition for pigmented coatings. More particularly, it relates to binding compositions comprising said reaction product and an elastomeric or resin latex which may additionally contain up to about 25% by weight of ammonium zirconyl carbonate.
In certain types of coated fine papers the chief types of binders used for paper coatings are casein and soy protein, although styrene-butadiene latices, acrylic latices, polyvinyl alcohol, certain types of starch products, etc., have also been used. However, because of its versatility and the fact that it is easily insolubilized, casein has heretofore been the preferred pigment dispersion medium, particularly for high grade paper coatings. Unfortunately, the high cost and food product applications of casein have limited its use as a paper coating adhesive. As to soy protein binders, the raw material for this product is abundant; however, the availability of a manufactured soy bean isolate is limited.
Efforts to substitute other materials for casein and soy protein have had varying degrees of success. Because of its ready availability and relative cheapness, starch products have in recent years received much attention for possible use in binders for paper coating compositions.
All starches contain two types of molecular structures: an "amylose" fraction which is essentially linear long chain material, and an "amylopectin" fraction which is a highly branched material. Varieties of starch have been developed, i.e., waxy starches, which contain a high percentage of the amylopectin fraction (&gt;90%); these products are referred to as "high amylopectin starches" and are somewhat more expensive than ordinary starch which has a higher amylose content.
U.S. Pat. No. 4,243,564, pertinent portions of which are incorporated herein by reference, describes the use of high amylopectin starches to prepare a reaction product of hydrolyzed styrene maleic anhydride copolymer with a dispersed high amylopectin starch, said product being neutralized with a volatile alkali such as ammonium hydroxide. Coating compositions containing said reaction product as a binder constituent are said to provide satisfactory coating compositions for use in paper coatings and paints. U.S. Pat. No. 4,243,564 teaches that the key to satisfactory coating compositions is the use of a high amylopectin starch, rather than an ordinary starches.
Although considerably cheaper, and more readily available, than casein or soy protein, high amylopectin starches are nevertheless more costly than ordinary starches. Furthermore, it has been found that the use of high amylopectin starches in the manner taught by U.S. Pat. No. 4,243,564, to prepare paper coatings also requires that about 20% (by weight of the entire binder composition) of a cross-linking agent such as ammonium zirconyl carbonate be used. It would be advantageous to be able to use ordinary starches, rather than a high amylopectin starch if satisfactory coatings can be prepared therefrom. Furthermore, it would be desirable also to eliminate, or at least substantially reduce, the amount of cross-linking agent required.